The present invention relates to the recording and reproduction of audio signals that are modulated onto a plurality of adjacent or juxtaposed tracks on a recording medium. More particularly, the invention relates to the reduction of crosstalk interference components from one or more adjacent tracks during the reproduction of a desired track.
Although the invention will be described in the context of a helical scan video tape recorder system employing frequency modulated audio recorded along with frequency modulated video in a series of slant tracks along an elongated magnetic tape, the invention is applicable to other recording media in which audio signals are frequency, phase or amplitude modulated on to adjacent or juxtaposed tracks or bands.
Nearly all video tape recorders have employed the technique of frequency modulating video signals and recording them, using a head or heads carried by a high-speed rotating disc or drum, in tracks generally crosswise or at a slant angle to the longitudinal movement of the magnetic tape. Generally, audio information has been recorded longitudinally along the tape edge by a fixed head without using any modulation or encoding.
As video tape recorders have advanced in design smaller and smaller width tapes have been employed, slower linear speeds have been used and the spacing between tracks has been reduced such that each is directly adjacent another, causing video crosstalk from track to track. In two-head devices where each head reads every other track, video crosstalk has been reduced by using different head azimuths for adjacent tracks. However, the staggered azimuth technique is less effective for suppressing crosstalk in audio signals recorded along with video signals on the same tracks but at a frequency lower than the video signal spectrum. Thus, in video tape recorder systems in which it is desired to record audio signals by scanning them on to the slant tracks along with the video signals (in place of or in addition to the longitudinal audio track), it is necessary to employ further techniques to suppress audio crosstalk from track to track.
Before summarizing the way in which the present invention solves the above defined problem, it is useful to refer generally to a few basic principles of frequency modulation.
In frequency modulation, the ratio of the frequency deviation, f.sub.d, of the carrier, f.sub.c, to the modulating frequency, f.sub.m, is known as the modulation index, m. That is, EQU m=(f.sub.d /f.sub.m)
FIGS. 1a through 1d show the sidebands resulting from various values of m. Consider the simplest case of a low value, m=1/2. There are then only two sidebands, one each side of the carrier at frequencies f.sub.c +/-f.sub.m. When a signal containing these three frequency components (in appropriate phase relationship) is applied to an FM demodulator, the output will be a sine-wave at frequency f.sub.m.
FIG. 2a shows a signal in which only one of the sidebands is present; this can be considered a single sideband carrier modulated at frequency f.sub.m with a combination of frequency and amplitude modulation. If this signal is passed through a limiter to remove the amplitude variations (as is usual in FM systems), a second sideband appears (and the first is diminished in amplitude); see FIG. 2b. If this limited signal is fed to the demodulator, the output will again be a sine-wave of frequency f.sub.m. Thus any interfering signal, which can be considered as the equivalent of a single sideband will, after limiting and FM demodulation, give rise to an output at a frequency equal to the spacing between the carrier and the interfering signal. If the single sideband is large enough, limiting to remove AM will introduce the further sideband as above, giving a spectrum with the appearance of FIG. 2b, but with the wrong amplitude and phase relationships for demodulation to deliver a pure sine-wave. In this case the output will be a distorted wave with a fundamental frequency equal to the spacing, and harmonics (both even and odd) dependent on the amplitude of the single sideband. This interference is proportional to EQU f.sub.i (a cos 2.pi.f.sub.i t-a.sup.2 cos 4.pi.f.sub.i t+a.sup.3 cos 6.pi.f.sub.i t- . . . )
where f.sub.i is the separation between the carrier and the single sideband, and a is the ratio of the amplitudes of the single sideband to the carrier.
It is apparent that the magnitude of this is directly proportional to f.sub.i, and that for small values of a, the fundamental frequency f.sub.i is dominant. For example, if a=0.1, the wave contains 0.01 or 1% of 2nd harmonic, and 0.001 or 0.1% of 3rd.
Suppose frequency modulation with a carrier f.sub.c is used in a scanned system, and that there is crosstalk between scans. In the absence of modulation but presence of slight drift in f.sub.c or slight variations in writing or reading speed, the crosstalk from an adjacent scan can be considered as a single sideband whose spacing from the f.sub.c of the current scan is small. Hence, after demodulation, the interference caused by the crosstalk will consist of low frequencies, and their magnitude will be low. However, when the carrier, either for the current scan or the adjacent one, or both, is modulated, the crosstalk can be considered as many single sidebands (that is, not appearing as pairs of sidebands symmetrically placed each side of the carrier), and the demodulated inerference will contain many frequencies.
If the deviation of the carriers is large compared with audio frequencies, the demodulated interference will cover the whole audio spectrum. However, it appears that the magnitude of each spectral line in the demodulated interference will still be proportional to its frequency, and thus higher frequencies will be more significant. In a practical scanned system in which the crosstalk from preceding and/or succeeding scans using the same nominal carrier frequency f.sub.c is at least 10 dB down compared to the current scan, the interference in the absence of modulation is likely to be negligible, since it will be a very low amplitude signal at a few tens of Hz where the human ear is insensitive, but the non-linear distortion caused by the interferences in the presence of modulation is likely to be intolerable. The interference extends across the whole audio frequency range because the spectrum of the FM signal on the current scan overlaps (in fact substantially coincides) with that leaking from adjacent scans.
The interference from one scan to another is similar to that caused by the multipath propagation of over-the-air FM signals. Rigorous analyses of FM interference is presented in Chapter 11 of Modulation, Noise and Spectral Analysis by Philip F. Panter, McGraw-Hill, San Francisco, 1965.
It is instructive to consider the addition of a compression-expansion noise reduction system. As shown above, a sideband in the interfering crosstalk gives rise to a demodulated tone whose amplitude is proportional to the amplitude of the sideband and to the frequency spacing between the sideband and the wanted carrier. In general, as the percentage modulation is increased, the amplitude of the sideband increases. Hence compression, which increases the percentage modulation for low output levels, will automatically also increase the demodulated interference. Expansion will reduce it again, but only to about the same level as it would have had without compansion, since the interference is usually in the same part of the audio spectrum as the modulating signal. Therefore, if the nonharmonic distortion resulting from interference is intolerable without noise reduction it is unlikely to be tolerable with it. Thus, the addition of a compression-expansion noise reduction system is, in itself, not helpful in solving the problem of interfering crosstalk.
In seeking a solution to the crosstalk interference problem, a practical constraint in a video tape recorder system, and no doubt in other recording systems, is that only a limited bandwidth is available for recording the frequency modulated audio signal. For example, in the case of a "color under" system, the video chrominance information is modulated on to a carrier at a frequency below the main frequency modulated luminance information. A relatively small portion of the recordable frequency spectrum is available for the frequency modulated audio.
It is therefore an object of the present invention to reduce crosstalk interference components from one or more adjacent tracks in recording and reproducing systems in which audio signals are modulated and recorded on to a plurality of adjacent tracks on a recording medium.
It is a further object to reduce crosstalk interference components from one or more adjacent tracks in videotape recording and reproducing systems in which audio signals are modulated on to a carrier and recorded along with modulated video signals on to a plurality of adjacent tracks or scans on a recording mediums.
It is yet a further object of the present invention to reduce crosstalk interference components from one or more adjacent tracks in recording and reproducing systems in which audio signals are modulated and recorded on to a plurality of adjacent tracks on a recording medium such that the reduction in crosstalk interference components is achieved without a significant increase in recorded bandwidth of the modulated audio signals and without a significant degradation of the signal-to-noise ratio of the reproduced audio signals.
These and other objects of the invention will be better understood as the following description is read and understood in connection with the drawings.